W126 ABS Problem
 

The car is a 1988 420SEL with 53k miles. The ABS warning light comes on after each startup, usually within 2 or 3 minutes of driving around, and often after hitting a bump. The light does not go off until the car is shut off and re-started. The ABS does not function.

I had my regular mechanic try and find the problem, however he does not have the computer and code reading software for the 126 chassis. He did, however, hook my car up to his computer as if it was a 140 chassis (which it isn't) and the error code indicated that the control unit had a bad valve?, and thus, the entire control unit would need to be replaced. New this part is $1000, used $200. I'm referring to the unit on the driver's side of the engine bay with "ABS" enscribed on its black plastic cover.

I can either take the car to a MB dealer and have them diagnose the problem (which will be at least $100), or I can just try putting the used ABS control unit in. My mechanic said he has never seen this part go bad before, however there's always a first...

Any suggestions? Does anyone feel confident that error codes for the 126 would apply to the 140 as well, or vice-versa?

Thanks in advance.

The symptoms are for a bad front speed sensor! Have them tested before going after the computer.
Using a volt/ohm meter at the connections should do.

The system has NO diagnostics except volt/ohm meter, pin by pin.

I second the motion on speed sensors. The early systems are tested with the Bosch LED Tester.

Can you outline the procedure for testing the speed sensors?

Thanks for the help.

Basic testing is easy. Basic testing won't find your problem.

As a Bosch Service Center we have had the Bosch LED Tester since 1986. It is the tool for pre OBD (on board diagnostics) ABS.

You unplug the controller and plug in the tool. It instantly, with LEDS, indicates status of B+, ground, sensor continuity, and state of internal diode. It has the ability to control the pump. It has the ability to release applied brakes and apply released brakes AND through an analog gauge it gives dynamic readings of the wheel speed sensors.

The prescribed way to evaluate is to place car on lift and rotate each wheel and view the reading on the meter. That doesn't fix cars either. What it takes to find the sensor problems (the problems are of very slight deviations - those that the controller picks up on) is to ignore the rule (do not drive while the tester is installed). While driving at about 2mph the switch is moved back and forth from sensor to sensor. By seeing the output in exactly the same condition (all wheels going 2mph) a accurate comparison can be made. It is infallable in execution. We have never missed a call on one since we figured out how to do it this way. The way the tool is designed: to rotate, by hand the individual wheels only picks out the easy ones.

I tell you this to give a feel if you are interested in testing the way most people do it (those without the designated tester). Most techs use an oscilliscope to watch the output of the sensors. This probably would normally be done with a BOB (break out box). This device would allow all the sensors to be electrically coupled to a multi channel scope. One will see a sign wave AC voltage with rising amplitude and frequency with speed. The amplitudes of the front two sensors should be even at even road speed. Testing should be done at low speed as the greatest differences appear there. At low speed each inducted pulse can be reviewed. by viewing them all at once comparisons can be made.